From DE-OS 1 941 808 and DE-OS 1 941 809 a tape-suspended meridian gyro is known wherein the excursion of the gyro from a tape zero position is detected by a pick-off and is converted into an electrical signal. This signal is supplied through an amplifier to a torquer which acts about the tape axis. The torquer acts on the gyro and compensates for the "gyro directional torque" which acts about the tape axis on the gyro and endeavors to align the spin axis with north. The energizing current of the torquer is proportional to the gyro controlling torque. A display supplied by this energizing current is provided for indicating the north direction. In the mentioned printed publications this display device consists of a servomotor by which the tape suspension is rotated until the energizing current becomes zero. Then, the spin axis is aligned with north such that the gyro directional torque is no longer effective. A telescope is connected to the tape suspension which is then aligned with north.
DE-OS 2 229 431 shows a tape-suspended meridian gyro in which the excursion of the gyro from a tape position of rest is also detected under the influence of the gyro directional torque by means of a pick-off and is converted to an electrical signal. Through an amplifier this signal controles a servomotor which rotates a telescope. A position pick-up is connected to the servomotor. Through an amplifier, the signal of the position pick-up determines the energizing current of a torquer acting about the tape axis on the gyro.
Finally, DE-AS 21 24 357 shows a meridian gyro apparatus having a gyro suspended by a tape in an intermediate housing. The pick-up generates a pick-off signal according to the excursion of the gyro from its tape zero position. The torquer exerts a counter-torque on the gyro, counteracting the gyro directional torque, with the pick-off signal again being supplied with a high gain to the torquer. The intermediate housing can be rotated by a servomotor. The pick-off signal is supplied to a memory. By means of a control circuit, after the adjustment of the gyro, the pick-off signal can be picked-off from the pick-off and can be supplied to the servomotor through a switching device. A feedback loop makes the angle of rotation of the servomotor proportional to the memorized pick-off signal.
The above mentioned meridian gyros allow the relatively quick determination of the north direction. The meridain gyros are arranged as free as possible from disturbances on a support which in turn, fixedly stands on the ground and is, as much as possible, shielded from vibrations. It is impossible to arrange such meridian gyros without any further measures in a disturbed environment, a vehicle, for example.
Therefore, filter arrangements have been provided by which the signal representing the "deviation from north", i.e. the energizing current of the torquer, is filtered.
DE-OS 25 45 026 shows a tape-suspended gyro with pick-off and torquer in which a signal proportional to the energizing current of the torquer is digitized by an analog-to-digital converter. The digital signal received with a fixed clock is supplied to a computer for forming recursive mean values. Each of the last recursively received mean values is directly output. A filtering is effected by forming the mean values. The recursive forming of this mean value ensures that a value for the deviation from north, even if it is quite an inaccurate value, is quickly present, and is corrected more and more in the course of time, the necessary memory capacity remaining within limits.
DE-OS 26 18 868 shows a similar arrangement. There, the mean value is formed from a fixed number of digital signals, i.e. the last N digital signals, with N being a fixed interger. This mean value is also recursively formed.
The generation of the mean values allows the averaging of white noise. However, in meridian gyros of the present type, in a disturbed enviroment, accelerations at the tape suspension cause relatively slowly increasing and again decreasing changes of the energizing current of the torquer which cannot be eliminated by forming the mean. On the contrary, the mean value is falsified by such disturbances.
From DE-PS 31 31 110 a gyro apparatus with a tape-suspended meridian gyro is known in which the gyro directional torque is compensated for by a counter-torque. The counter-torque is exerted by a torquer acting about the tape axis on the tape-suspended gyro housing. The torquer is supplied through an amplifier by a pick-off which responds to excursions of the gyro housing relative to a housing reference. In order to suppress disturbances which appear by a "rectifier effect" with oscillations of the apparatus housing, two acceleration detectors with their sensitivity axes perpendicular to each other are arranged at the apparatus housing. Signal processing means comprise a model for the disturbing dynamics according to which accelerations act as disturbances about the vertical axis. The signals of the accelerometer are supplied to this model. The model provides a disturbance signal whereby the measurement of the gyro directional torque is corrected.
In the meridian gyro according to DE-PS 31 31 100 the acceleration is measured at the tape suspension. This requires additional accelerometers. The received measuring value is corrected by a value which is obtained from a model only and represents the influence of the disturbance.
From DE-PS 31 31 111 a gyro apparatus for determining the north direction is known which also comprises a tape-suspended meridian gyro, a pick-off and a torquer similar to the above mentioned gyro apparatus. In order to suppress disturbances, a Kalman filter is provided which imitates the system of the restrained meridian gyro and, as the first filter input signal, obtains the difference from the signal supplied to the torquer (energizing current) and an estimate of this signal provided by the Kalman filter. As a second filter input signal, the Kalman filter additionally receives the difference of a signal representing the deviation of the meridian gyro from the housing reference and an estimate of this signal provided by the Kalman filter. Then, the estimate of the energizing current is output as a measure of the deviation of an apparatus reference from the north direction. Thereby, it shall be achieved that the estimate is quickly corrected to a "true" value and the disturbances are suppressed.
However, the Kalman filter provided in DE-PS 31 31 111 suppresses only stochastic disturbances but not disturbances of the above mentioned type which relatively slowly increase and then correspondingly slowly decrease again.